I have trawled through Q(u)enya verbs in Eldamo to see if I can glean any idea how derived verbs form their frequentatives. I have identified the following ones, are there any others I have missed?
tintila-/tintina- (not sure if the freq. is coming from the base verb or the causative here)
- naina-/nainaina- (both glossed the same so may not be a tru freq.)
- sirya-/sisíria- (the latter is unglossed but presumable is a freq. of the former, but there's also the consonantal sir- so the case is ambiguous)
- tanta-/tantila- (this is EQ but in line with tinta-/tintila-)
- ororia- (this was deleted)
So plain a-verbs and ya-verbs seem more or less follow the pattern seen in consonantal verbs (stem duplication, doubled/strengthened final consonant or lengthened sundóma). Whether
tintila- tantila- is indicative of how ta-verbs form their frequentatives, hard to say.
It might be also possible to employ the prefix vor(o)- to form a frequentative of a verb.